Tannins are plant-derived polyphenolic anti-nutritional factors that impair nutrient absorption, damage the intestinal barrier, and disrupt metabolic homeostasis. Some animals, however, have an innate resistance to the effects of tannins. Sika deer (Cervus nippon) exhibit strong tolerance to long-term consumption of tannin-rich oak plants, and Akkermansia muciniphila is considered a key functional bacterium, yet the underlying mechanism of this remains unclear. In this study, we transplanted fecal microbiota from tannin-acclimated sika deer into antibiotic-pretreated C57BL/6J mice, followed by tannin gavage to establish a stress model. Growth performance, serum glycolipid indices, and hepatic metabolic gene and protein expression were detected. Results showed no significant differences in body weight, fat deposition, or serum biochemical parameters between groups. In the transplantation group, key genes involved in glycolysis, detoxification, and lipid metabolism were significantly upregulated, while CYP7A1, a bile acid synthesis rate-limiting gene, was significantly downregulated at both transcriptional and protein levels. Akkermansia was specifically enriched in the gut of transplanted mice. In conclusion, the microbiota of the sika deer gut alleviates tannin toxicity and enhances host tolerance by regulating hepatic glycolipid and bile acid metabolism pathways. The results of our study provide new insights into the adaptation mechanisms of herbivores to high-tannin diets.